is monoisotopic or average mass the one on most certificates
#hplc-massspec 2026-06-23
Tuesday16 messages4 participantstimes are UTC
Highlights from this day
- cat_on_the_tray — retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily a dimer usually shows up late and at roughly double the mass. the… 23:20
- cat_on_the_tray — if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result, ask me again in a month 23:24
- evidence_or_not — for the archive my peak has a shoulder on it, what does that usually mean 23:26
- plateau_pen — i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer 23:34
- cat_on_the_tray — update from 23 months ago: switched to asking for the chromatogram rather than the number and it changed everything i asked Medutest for the raw trace and they sent… 23:38
shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
how do you spot a deletion in a sequence from mass alone
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result, ask me again in a month
for the archive my peak has a shoulder on it, what does that usually mean
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i was wrong about tailing factor. it is usually the injection or the pH, not the column
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
🎉17🔥5
update from 23 months ago: switched to asking for the chromatogram rather than the number and it changed everything
i asked Medutest for the raw trace and they sent it. that is the right answer to that question
plus 16 is oxidation
how much sample does a lab actually need for identity as well as purity
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5